1. Field of the Invention
The present invention relates generally to a digital down-converter, and in particular, to a digital down-converter for sampling a received radio signal with a radio frequency (RF) signal or an intermediate frequency (IF) signal and then performing digital signal processing on the sampled signal.
2. Description of the Related Art
A conventional digital down-converter (DDC), such as a typical frequency down-converter for a digital signal processing circuit in a data communication receiver, will be described with reference to FIG. 1. Referring to FIG. 1, an input signal 100, such as an RF or IF modulation carrier, is an IF signal of a frequency Fif1, sampled with a frequency Fs1. For detection, the IF input signal 100 is mixed with a cosine wave c(t) and a sine wave −s(t) of a frequency Fc output from a local oscillator (LO) 102 by a mixer 101a and a mixer 101b. When the frequency Fc for the cosine wave and the sine wave, output from the local oscillator 102, is set to satisfy a relationship of Fc=Fif1, the IF input signal 100 is converted to a detection process frequency signal. The converted IF signals are subjected to 1/n down-sampling (Fs2=Fs1/n) by sampling rate converters 103a and 103b, creating baseband signals of a frequency Fb. The baseband signals are rolloff-shaped by rolloff filters 104a and 104b, and then, variably amplified by undepicted automatic gain control (AGC) amplifiers. As a result, the IF input signal is output as a complex baseband signal comprised of an in-phase component I signal and a quadrature-phase component Q signal.
As stated above, in the digital down-converter (DDC) for sampling a received signal with a radio frequency (RF) or an intermediate frequency (IF) and converting the sampled digital signal to a complex baseband signal by digital signal processing, a spurious signal generated by an operation of the local oscillator 102 used by the mixers 101a and 101b deteriorates an adjacent channel interference characteristic or an outband interference characteristic. This is because an adjacent channel or an outband signal is multiplied by the spurious signal, thus generating undesired signals outside the corresponding band.
To solve this problem, countermeasures have been taken by (1) increasing an operation precision to decrease a level of the spurious signal from the location oscillator 102 to a tolerable level, (2) adopting a dither technique to reduce a spurious peak level by spreading the spurious signal generated from the local oscillator 102, and (3) using a spurious-free frequency based on the fact that generation of the spurious signal by the local oscillator 102 depends upon an oscillation frequency of the location oscillator 102.
However, even the countermeasures for the spurious signal (1) cause an increase in circuit scale and power consumption; (2) cause an increase in circuit scale and power consumption, though less than when the measurement of decreasing the spurious level has been taken, and a deterioration in a carrier-to-noise ratio (C/N); and (3) cause the restriction of available frequencies. In particular, since the power consumption of the local oscillator 102 requiring the maximum processing speed to operate with a sampling frequency of the received signal holds a large part of the total power consumption of the digital down-converter, the increase in power consumption of the local oscillator 102 is very undesirable.